The present invention relates to Roots-type blowers, and more particularly, to such blowers in which the lobes are not straight (i.e., parallel to the axis of the rotor shafts), but instead, are “twisted” to define a helix angle.
Conventionally, Roots-type blowers are used for moving volumes of air in applications such as boosting or supercharging vehicle engines. As is well known to those skilled in the art, the purpose of a Roots-type blower supercharger is to transfer, into the engine combustion chambers, volumes of air which are greater than the displacement of the engine, thereby raising (“boosting”) the air pressure within the combustion chambers to achieve greater engine output horsepower. Although the present invention is not limited to a Roots-type blower for use in engine supercharging, the invention is especially advantageous in that application, and will be described in connection therewith.
In the early days of the manufacture and use of Roots-type blowers, it was conventional to provide two rotors each having two straight lobes. However, as such blowers were further developed, and the applications for such blowers became more demanding, it became conventional practice to provide rotors having three lobes, with the lobes being twisted. As is well known to those skilled in the art, one of the distinguishing features of a Roots-type blower is that it uses two identical rotors, wherein the rotors are arranged so that, as viewed from one axial end, the lobes of one rotor are twisted clockwise while the lobes of the meshing rotor are twisted counter-clockwise. As is now also well known to those skilled in the art, the use of such twisted lobes on the rotors of a blower, of the type to which the invention relates, results in a blower having much better air handling characteristics, and producing much less in the way of air pulsation and turbulence.
An example of a Roots-type blower is shown in U.S. Pat. No. 2,654,530, assigned to the assignee of the present invention and incorporated herein by reference. Many of the Roots-type blowers which are now used as vehicle engine superchargers are of the “rear inlet” type, i.e., the supercharger is mechanically driven by means of a pulley which is disposed toward the front end of the engine compartment while the air inlet to the blower is disposed at the opposite end, i.e., toward the rearward end of the engine compartment. In most Roots-type blowers, the air outlet is formed in a housing wall, such that the direction of air flow as it flows through the outlet is radial relative to the axis of the rotors. Hence, such blowers are referred to as being of the “axial inlet, radial outlet” type. It should be understood that the present invention is not absolutely limited to use in the axial inlet, radial outlet type, but such is clearly a preferred embodiment for the invention, and therefore, the invention will be described in connection therewith.
A more modern example of a Roots-type blower is shown in U.S. Pat. No. 5,078,583, also assigned to the assignee of the present invention and incorporated herein by reference. In Roots-type blowers of the “twisted lobe” type, one feature which has become conventional is an outlet port which is generally triangular, with the apex of the triangle disposed in a plane containing the outlet cusp defined by the overlapping rotor chambers. Typically, the angled sides of the triangular outlet port define an angle which is substantially equal to the helix angle of the rotors (i.e., the helix angle at the lobe O.D.), such that each lobe, in its turn, passes by the angled side of the outlet port in a “line-to-line” manner. In accordance with the teachings of the above-incorporated U.S. Pat. No. 5,078,583, it has been necessary to provide a backflow slot on either side of the outlet port to provide for backflow of outlet air to transfer control volumes of air trapped by adjacent unmeshed lobes of the rotor, just prior to traversal of the angled sides of the outlet port. Although the present invention is not limited to use with a blower housing having a triangular outlet port in which the angle defined by the angled side corresponds to the helix angle of the rotors, such an arrangement is advantageous, and the invention will be described in connection therewith.
As is now well known to those skilled in the art, and as will be illustrated in the subsequent drawings, a Roots-type blower has overlapping rotor chambers, with the locations of overlap defining what are typically referred to as a pair of “cusps”, and hereinafter, the term “inlet cusp” will refer to the cusp adjacent the inlet port, while the term “outlet cusp” will refer to the cusp which is interrupted by the outlet port. Also, by way of definition, it should be understood that references hereinafter to “helix angle” of the rotor lobes is meant to refer to the helix angle at the pitch circle of the lobes.
One of the important aspects of the present invention relates to a Roots blower parameter know as the “seal time” wherein the reference to “time” is a misnomer, as the term actually is referring to an angular measurement (i.e., in rotational degrees). Therefore, “seal time” refers to the number of degrees that a rotor lobe (or a control volume) travels in moving from through a particular “phase” of operation, as the various phases will be described hereinafter. In discussing “seal time” it is important to be aware of a quantity defined as the number of degrees between adjacent lobes, referred to as the “lobe separation”. Therefore, in the conventional, prior art Rootstype blower, having three lobes, the “lobe separation” (L.S.) is represented by the equation: L.S.=360/N and with N=3, the lobe separation L.S. is equal to 120 degrees. There are four phases of operation of a Roots-type blower, and for each phase there is an associated seal time as follows: (1) the “inlet seal time” is the number of degrees of rotation during which the control volume is exposed to the inlet port; (2) the “transfer seal time” is the number of degrees of rotation during which the transfer volume is sealed from both the inlet “event” and the backflow “event”; (3) the “backflow seal time” is the number of degrees during which the transfer volume is open to the “backflow” port (as that term will be defined later), prior to discharging to the outlet port; and (4) the “outlet seal time” is the number of degrees during which the transfer volume is exposed to the outlet port.
Another significant parameter in a Roots-type blower is the “twist angle” of each lobe, i.e., the angular displacement, in degrees, which occurs in “traveling” from the rearward end of the rotor to the forward end of the rotor. It has been common practice in the Roots-type blower art to select a particular twist angle and utilize that angle, even in designing and developing subsequent blower models. By way of example only, the assignee of the present invention has, for a number of years, utilized a sixty degree twist angle on the lobes of its blower rotors. This particular twist angle was selected largely because, at that time, a sixty degree twist angle was the largest twist angle the lobe hobbing cutter then being used could accommodate. Therefore, with the twist angle being predetermined, the helix angle for the lobe would be determined by applying known geometric relationships, as will be described in greater detail subsequently. It has also been known in the Roots-type blower art to provide a greater twist angle (for example, as much as 120 degrees), and that the result would be a higher helix angle and an improved performance, specifically, a higher thermal compressor efficiency, and lower input power.
As is also well known to those skilled in the art, and as will be described in greater detail subsequently, the air flow characteristics of a Roots-type blower and the speed at which the blower rotors can be rotated are a function of the lobe geometry, including the helix angle of the lobes. Ideally, the linear velocity of the lobe mesh (i.e., the linear velocity of a point at which meshed rotor lobes move out of mesh) should approach the linear velocity of the air entering the rotor chambers through the inlet port. If the linear velocity of the lobe mesh (referred to hereinafter as “V3”is much greater than the linear velocity of incoming air (referred to hereinafter as “V1”), the result will be that the movement of the lobe will, in effect, draw at least a partial vacuum on the inlet side. Such a mismatch of V1 and V3 will cause pulsations, turbulence and noise, (and creating such requires “work”), all of which are serious disadvantages on an engine supercharger, rotating at speeds of as much as 15,000 to about 18,000 rpm.
Those skilled in the art of Roots-type blower superchargers have, for some time, recognized that it would be desirable to be able to increase the “pressure ratio” of the blower, i.e., the ratio of the outlet pressure (absolute) to inlet pressure (absolute). A higher pressure ratio results in a greater horsepower boost for the engine with which the blower is associated. The assignee of the present invention has utilized, as a design criteria, not to let the Roots-type blower exceed a pressure ratio which results in an outlet air temperature in excess of 150 degrees Celsius.